l3build: re-instate \PACKAGE and \CLASS, add .cfg for 2e use

git-svn-id: http://www.latex-project.org/svnroot/experimental/trunk@5054 de43f980-851b-0410-b2f7-c40aca1f87e0
l3build: add a test to test the test system

I'm in a bit of a rush, so I didn't try running the testfiles
from the main build.lua file; rather added a new one just for
tests. N.B. it takes regression-test.tex from l3build/ not
from ../support/

I'm using %&pdftex to run the test file in plain;
is that the right idea?

git-svn-id: http://www.latex-project.org/svnroot/experimental/trunk@5055 de43f980-851b-0410-b2f7-c40aca1f87e0